Ink jet printers, in particular drop-on-demand (DOD) or impulse printers having ink jet print heads with acoustic drivers to accomplish ink drop formation, are well known in the art. Some ink jet print heads eject ink from the print head in a direction perpendicular to the plane of one or more ink pressure chambers, and other print heads eject ink in a direction parallel to the plane of one or more ink pressure chambers.
The principle underlying the successful operation of an ink jet print head of the DOD type is the manipulation of pressure within an ink pressure chamber to achieve controlled emission of ink droplets from the print head through one or more orifices. In general, a DOD ink jet print head, having an ink pressure chamber coupled to a source of ink and an ink drop ejecting orifice, is operated as set forth below. An acoustic driver expands and contracts the volume of the ink pressure chamber to eject a drop of ink from the orifice. The acoustic driver applies a pressure wave to ink residing within the ink pressure chamber to cause the ink to pass outwardly through the orifice in a controlled manner.
Ink jet print heads generally have a layered structure including a plate having a plurality of orifices through which ink is deposited onto a print medium. The structure of this orifice plate is important to the efficiency of the print head and the quality of printed images produced thereby. During the orifice plate manufacturing process, burrs or other surface defects are sometimes formed at the edges of orifices. Such defects impede the flow of ink through the orifice plate and can cause "off-axis shooting" (i.e., deposition of ink onto the print medium at a location that is not aligned with the orifice). In addition, such defects can cause the effective orifice size to decrease by obstructing some portion thereof. Such obstruction results in slowed, inefficient ink droplet deposition.
Off-axis shooting is a problem that affects the print image's quality and manifests itself in several ways. Since the ink or coloring agent can be directed to a spot on the target media offline from the intended strike location, some areas of the target media can be uncolored, appearing as white spots that give the appearance to the naked eye of bands in the printed image. Lines in the printed image can also be printed in a form that is not straight, appearing as wiggly or fuzzy lines. These manifestations can also be created by the formation of satellite drops of ink as the ink is ejected from the nozzles of the ink jets with surface defects, resulting in the formation of multiple droplets that impact the target medium in an unintended and undesired pattern.
Another manifestation of the off-axis shooting problem is the change in the shading of the secondary colors of the printed image because of either the lack of a drop of ink of a particular color or different volumes of inks of a specific color being delivered to different locations on the printed image.
Similarly, off-axis shooting can be caused by the orifices being obstructed during operation or maintenance of the print head. For example, small slivers of metal, as small as 5 microns, on the surface of an 80 micron nozzle orifice as a result of the fabrication process can serve directly to obstruct the nozzle orifice, especially when oriented along the axis of the orifice opening, or indirectly can cause obstructions as nucleation sites for the growth of ink crystals because inks of different colors are sometimes incompatible and form a solid precipitate when mixed together. Such mixing can occur inadvertently during a print head maintenance procedure in which a wiper blade moves across the print head, mixing together all of the inks. The mixture of incompatible inks, for example black and magenta, can be held in place on the small surface defects and continue to build up during continued operation of the printer. Since oxygen is essential for crystal growth and the print head is exposed to air, the build-up can continue until it results in the formation of sufficient solids that can cause off-axis shooting or ultimately in a catastrophic failure of the print head.
These problems are solved in the print head design and process of treating the orifice plate in the print head design of the present invention.